crypto/rsa: refactor PKCS#1 v1.5 signature and verification

crypto/rsa: refactor PKCS#1 v1.5 signature and verification

VerifyPKCS1v15 doesn't need to be constant time and can do the safer and
simpler construct-and-compare.

Updates #67043

Change-Id: I014cfd4485fad409c5f86be71488da63af25a584
Reviewed-on: https://go-review.googlesource.com/c/go/+/587278
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>